audioguru's FM transmitter, antenna design.

[rdljpl]

StereoCaster

I finally got my transmitter to produce a stronger signal. I built a 29" dipole antenna and connected it. Initially I started with 31 inches and was able to tune the transmitter better, and then I trimmed the antenna by cutting the wire to the final 29 inches.

My next project is to get it mounted in it's permanent location and put the antenna into the attic so maybe I can get better quality and a little better range. I was able to pick the signal up from the car radio at 150 to 200 feet, but it is still not as strong as I would like, and the quality is not there yet.

I think that some tweaking is in order. I did receive a diagram of a Dipole antenna at the end of an 8' 50 ohm coax. I am going to try that next.

rdljpl

 

[audioguru]

The range of my FM transmitter is more than 2km with just a piece of wire hanging as its antenna. I cannot measure its range properly because my FM dial is full of stations so I used the same frequency as a low power foreign language station on the other side of my city. The quality is excellent because I added pre-emphasis.
The cheap "FM radio" from The Dollar Store picks it up across the street. My Sony Walkman radio picks it up from a range of about 300m but overloads if it is closer.

 

[flat5]

"I removed the antenna and plugged it back in and noticed an instant of clarity so I pulled it back to the point where it was no longer making contact and the output quality improved and the noise stopped. I found that by moving the antenna close to making contact with the circuit it would work pretty well but the further I moved it away the worse it got."
You were adjusting the "coupling". You might want to Google that. It is often (or used to be) done by adjusting the space between two coils or by adjusting a "tap" on one coil.

 

[audioguru]

I think your FM radio is cheap and is easily overloaded when the transmitter works properly. My cheap Sony Walkman radio gets overloaded so the transmitter shows up all over the dial.

 

[shawkat kuchar]

thinkssssssssssss

 

[rfs100]

!!!Is all this legal??

Not to be the 'ol Fuddy Duddy chasing the kids off my front lawn, but isnt all this long-range, high power FM stuff illegal? Im not American, so I cannot really comment on all this...but Im certain that someone trying to get 2 miles of transmission on the FM band would incur a visit from the DOC, along with a stiff fine here in Canada.
I think everyone is getting all wrapped nup in the "coolness" of the subject matter, and not really helping this guy out by assisting in him doing something illegal.
Come on guys. shouldnt someone be helping out this guy in getting his Amateur Radio ticket, so he can learn how, and what he can broadcast legally.
In the good 'ol days o this forum, discussions about illegal activities would be squashed likea bug right off the bat, not allowed to go on for pages of assistance to break the law....someone asleep at the switch here?
The reason that people find that these FM transmitter kits have such crappy range,,,is because the FCC/DOC requires limited range as acceptance to them being sold.

 

[audioguru]

Not to be the 'ol Fuddy Duddy chasing the kids off my front lawn, but isnt all this long-range, high power FM stuff illegal? Im not American, so I cannot really comment on all this...but Im certain that someone trying to get 2 miles of transmission on the FM band would incur a visit from the DOC, along with a stiff fine here in Canada.

It is illegal only when you get caught interfering with a real radio station and somebody complains. Or maybe if you transmit profanity.
Maybe the old woman next door has a cheap radio that is overloaded by your transmitter even when it is on a different frequency. Then she complains and the RF cops will catch you.

When I was 18 I had a visit by the RF cops. I made a fan from an old sparking motor and it caused radio and TV interference all over the place. The cops were friendly and we talked about electronics. They said my fan was too dangerous (a wooden model airplane propeller that could easily cut off your arm) so they took away the old motor. Then my neighbours bought a new fan for me.

I used my FM transmitter only for one hour to measure its range and hear its excellent audio quality. I never tried it again.

 

[rfs100]

It is illegal only when you get caught interfering with a real radio station and somebody complains. Or maybe if you transmit profanity.
Maybe the old woman next door has a cheap radio that is overloaded by your transmitter even when it is on a different frequency. Then she complains and the RF cops will catch you.

When I was 18 I had a visit by the RF cops. I made a fan from an old sparking motor and it caused radio and TV interference all over the place. The cops were friendly and we talked about electronics. They said my fan was too dangerous (a wooden model airplane propeller that could easily cut off your arm) so they took away the old motor. Then my neighbours bought a new fan for me.

I used my FM transmitter only for one hour to measure its range and hear its excellent audio quality. I never tried it again.

Audioguru, first of all I want to say that while I am a SUPPOSED NEWBE around here, I have the utmost respect, and admiration for you, and your abilities and knowledge on electronics. We both grew up in an era where electronic magazines were filled with transmitter schematics of various outputs. Back then, the most that you could bother was a elderly radio with poor rejection, but nowsadays splattering rf across the broadcast bands just aint Koshure, and getting back to my main point, it is ILLEGAL modifying the transmitter to output anything other than its original output, to its original short wire wire antenna. I think that we should all consider helping people as though we live beside them, and that maybe you would not want all that RF and harmonics raining down on you on a daily basis from your neighbour. Again I would steer anyone interested in broadcasting to become a HAM, as they seem to be a dieing breed, and requirements are practically nonexistant now to become one.

 

[MRCecil]

Well put, rfs100.